Why SRB2 if SRB1 itself is capable of transfering NAS messages?

RAN
1

Hi Experts,
I was just reading some basics…
Why do you think SRB2 came into the picture when SRB1 itself is capable of transfering NAS messages?
Is it just to define a low priority SRB for NAS messages?

2

According to @Jaeku_Ryu (Sharetechnote):

“SRB1 is for RRC messages (which may include a piggybacked NAS message) as well as for NAS messages prior to the establishment of SRB2, all using DCCH logical channel”

http://sharetechnote.com/html/5G/5G_SRB.html

So NAS messages are transmitted over SRB1 till SRB2 is established in 5G.

3

But the same can be achieved by SRB1, why introduce another SRB?

4

Hi friends.
As mentioned SRB used to transfer RRC and NAS message.
I have 1 doubt: why SRB 1 used for RRC connection reconfiguration message transfer?
Why not SRB 2?
As I can see content of RRC connection reconfiguration having SRB 2 IE definition.

5

SRB2 is used to carry NAS messages has lower priority than SRB1

6

SRB2 is used when Security procedure is complete.

7

SRB 2 used for DL UL data transfer.

8

SRB1 can also be used for data transfer.
SRB1 is also used to encapsulate the NAS message if SRB2 has not been configured.

SRB2 has a lower priority than SRB1 and it is always configured after the security activation.

SRB0 uses the transparent mode RLC, while SRB1 and SRB2 use the acknowledged mode RLC.

9

Hi guys, what is the difference between srb1 and srb2 from the perspective of physical, mac and rlc layers?
Mmaybe an easy one but I could not find it.
What I find is srb1 is used for x messages and srb2 is used for y messages but not from the perspective of layer1 layer2?

10

Both SRB1 & SRB2 are mapped to DCCH logical channel.
So, RLC maintains separate logical channel and the transmission is in AM mode as opposed to TM mode followed for SRB0.
MAC & PHY are agnostic to the type of bearers (SRB/DRB).

11

Then is it true that from the perspective of mac layer there is no difference for SRB0, SRB1 and SRB2 but there is difference between SRB and DRB? Is this statement true?

12

What’s the difference between SRB vs DRB at MAC layer?
Both map to either DL-SCH/UL-SCH, right?

13

Yes but any other difference in MAC layer for SRB and DRB?

Or let me ask the question differently: Are signalling bearers applying the relevant configurarion on RLC layer only? Not MAC and physical layers, right?

14

I don’t think so.
Both SRB & DRB map to different logical channel and then they get multiplexed into SCH channel.
RLC does the segmentation/concatenation as per the transmission opportunity presented by MAC.

15

You can check LCID at mac layer indicates diff.

16

Thank you @ankgitm, but I am looking not an id based difference but rather behavioral difference.
When I check SRB default configurations, all are RLC layer configs.
So SRBs are RLC related configs, not lower layer based, right?

17

Basically SRB used logical channel for transfer for RRC and NAS message which are further mapped to transport and physical channel.